14: The EGR cooler uses coolant running through a set of passageways to cool the exhaust before reintroducing it into the engine. This process of recirculating exhaust air into the intake helps reduce tailpipe emissions. The factory EGR fails in two different ways and both generally stem from a failed oil cooler. The flow of coolant goes through the oil cooler, then directly to the EGR cooler. A clogged oil cooler means less and hotter coolant going to the EGR cooler, making it much less efficient. Generally, the EGR cooler will clog with soot or crack at the coolant passage, leaking coolant into the intake that could eventually lead to catastrophic engine failure.